Some of the very important biologically active compounds are either cyclic peptides or contain cyclic peptide moieties. Two outstanding examples that have been the subject of much study are the bracelet shaped cyclic depsipeptide valinomycin and the antibiotic actinomycin containing two cyclic peptide rings. Other examples of cyclic peptides are to be found in hormones, toxins, antitoxins and alkaloids. Cyclic peptides also have had great implications in recent studies of their linear counterparts, the proteins. They also serve as model compounds in the study of protein conformation under controlled and stringent conditions. Moreover, they serve as useful tools in the study of biochemical processes. The cyclic peptides occur with different ring sizes and also contains non-protein amino acids like N-methyl and D-amino acids. The studies of the three dimensional atomic arrangement of these compounds by x-ray crystallographic method, though few so far, have already yielded very valuable information of significant biological import. The aim of this project is to investigate by x-ray diffraction technique the detailed structure and conformation of a few selected cyclic peptides.